Coesite-bearing granulite retrograded from eclogite in Weihai, eastern China

Coesite in a granulite host-rock is reported from Weihai City, northeastern Shandong Province, China. Coesite occurs as inclusions in garnet, which is separated from the fine-grained matrix of quartz + clinopyroxene + homblende by a zoned corona of plagioclase + clinopyroxene + orthopyroxene + hornblende. Petrological evidence shows that the coesite-bearing granulite is retrograded from eclogite. Three stages in the metamorphic history of the granulite have been recognized. (i) Coesite-eclogite stage: The assemblage reconstructed from the inclusions in garnet is Mg-rich garnet + omphacite + coesite + rutile. (ii) During the granulite stage, these minerals reacted to form quartz + clinopyroxene + hornblende (in the matrix), clinopyroxene + albite (omphacite pseudomorphs) and plagioclase + clinopyroxene + orthopyroxene + hornblende (as corona around garnet). (iii) Amphibolite stage: Earlier minerals were replaced by biotite, epidote and hornblende. The P-T conditions estimated with various geothermobarometers show a T-increasing and P-decreasing path from the eclogite stage (T ∼ 720°C, P > 28 kbar) to the granulite stage (T ∼ 850°C, P ∼ 10 kbar). Thus the exhumation of the ultra-high-pressure rocks in Weihai proceeded under increasing temperature, and may not have been as fast as it was thought before